Sulfur compounds in liquid hydrocarbon fuels have been linked to serious human health and environmental problems. For example, sulfur is a well known poison for catalytic converters in motor vehicles and the resulting SOX in exhaust gas is a major contributor to air pollution. Emissions from poisoned catalytic converters may contain high levels of combusted hydrocarbons, oxides of nitrogen and/or carbon monoxide which, when catalyzed by sunlight, form ground level ozone, referred to as smog. Consequently, rigorous efforts are currently underway in both European countries as well as in the United States of America1,2 to reduce the sulfur content in liquid hydrocarbon fuels from the current levels of 300-500 ppmw total sulfur down to 15-30 ppmw (15 ppmw of total sulfur for diesel fuel and 30 ppmw total sulfur for gasoline). More recently, the need for ultra-low-sulfur fuels in fuel cell application is also getting more and more important—the total sulfur concentration in liquid fuel has to be reduced to less than 1 ppmw for proton exchange membrane fuel cell and below 10 ppmw for solid oxide fuel cell.
The U.S. Environmental Protection Agency has established stringent sulfur control programs for gasoline and diesel fuel. The Tier 2 gasoline sulfur control program started in 2004 requires that, by 2006, all U.S. gasoline must have an average sulfur content≦30 ppm. The Ultra-Low-Sulfur Diesel (ULSD) program begins in 2006 and requires that highway diesel fuel must have a maximum sulfur content ≦15 ppm. Consequently, deep desulfurization is becoming a great challenge for environmental chemists, the petroleum refining industry, and others. As used herein, “ultra-low sulfur diesel fuel” is defined as having a maximum sulfur content ≦15 ppm.
Ultra-low-sulfur diesel fuel (ULSD), less than 10 ppm total sulfur content in Europe (while ≦15 ppm in North America) has been produced in a hydrotreating facility based on SynTechnology at Scanraff, Sweden. Sweden was the first country to impose very strict quality specifications for diesel fuel, requiring a minimum 50 cetane number, a maximum of 10 ppm on sulfur content, and a maximum 5 percent on aromatics content. Other European countries are encouraging the early introduction of very-low-sulfur diesel fuel ahead of the shift to a European Union requirement for 50 ppm diesel in 2005. The United Kingdom and Germany have structured tax incentives for the early introduction of 50 ppm diesel fuel and have discussed incentives for introduction of a 10 ppm diesel fuel. BP's refinery at Grangemouth, United Kingdom uses a new higher activity AK30 Nobel catalyst (KF757) to produce 10 to 20 ppm sulfur diesel product. In the United States, Arco (BP Amoco) announced that it would produce a premium diesel fuel termed “EC Diesel” with less than 10 ppm at its Carson, Calif., refinery.
The production of ultra low sulfur diesel (ULSD) fuel for clean, complete combustion and low emissions is of great significance. Diesel fuel is used for many tasks. In agriculture, diesel fuels more than two-thirds of all farm equipment in the United States, because diesel engines can perform demanding work. In addition, it is the most widely used fuel for public buses and school buses throughout the United States. America's construction industry depends on diesel's power. Diesel engines are able to do demanding construction work, like lifting steel beams, digging foundations and trenches, drilling wells, paving roads and moving soil, safely and efficiently. Diesel also powers the movement of America's freight in trucks, trains, boats and barges; 94 percent of American goods are shipped using diesel-powered vehicles. No other fuel can currently match diesel in its ability to move freight economically.
The removal of sulfur-containing compounds from liquid fuels in the petroleum industry is currently achieved by hydrodesulfurization (HDS) with a Co—Mo/Al2O3 or Ni—Mo/Al2O3 catalyst or by an adapted HDS process (e.g. by improving catalytic activity, increasing the process severity such as hydrogen pressure or designing new reactor configurations). HDS is highly efficient in removing thiols, sulfides, and disulfides but is less effective for refractory sulfur-containing aromatic thiophene, benzothiophene and dibenzothiophene and their alkylated derivatives as depicted in FIG. 1, especially those containing functional groups that hinder the ring sulfur atoms (i.e., 4,6-dimethyl-dibenzothiophene). The difficulty in their removal may be due to their aromaticity (stability and inactivity) and the steric hindrance. For diesel fuel, it is very difficult for the current hydrotreating technology to reduce the sulfur content to less than 50 ppmw, because the remaining sulfur compounds in current diesel fuel with 500 ppmw total sulfur level are mostly the refractory sulfur compounds which are difficult to remove. Consequently, the sulfur content in gasoline cannot be reduced to less than 30 ppmw by current hydrotreating processes. The major problem for deep desulfurization of hydrocarbon fuels is that the current hydrotreating technology results in high hydrogen consumption and significant reduction of octane number due to olefin saturation. Particularly, desulfurization of fluid catalytic cracking (FCC) gasoline (containing 20-40 wt. % olefins) demands adsorbents of high selectivity to substantially reduce the amount of sulfur components while maintaining a minimum olefin saturation to provide a good octane number. In order to carry out deep removal of the sulfur compounds that account for only less than 1 percent of the fuel, 100 percent of the fuel needs to be processed at high temperatures under elevated pressures using hydrogen gas. The HDS process typically requires expensive catalysts, hydrogen gas, high-pressure equipment (up to 1,000 psig) and high-temperature (400-550° C.) to help produce environmentally-friendly low sulfur fuels.
Other approaches used to generate low sulfur fuels include costly biodesulfurization using bacteria that removes sulfur as a water soluble sulfate salt. In other approaches, sulfur oxidation using emulsified hydrogen peroxide has been pursued which requires extra post-processing to separate the oxidized sulfone (Unipure Corporation) and the Fischer-Tropsch process that produces non-oil-based synthetic diesel from natural gas. None of these approaches has been entirely satisfactory.
Sorbents have been also been developed to generate low sulfur fuels via sulfur adsorption. For example, Ag (I), Cu(I), Ni(II) and other transition metal exchanged Y zeolite-based adsorbents that allegedly work via π-complexation have been developed by two leading research groups of Song3 and Yang4 on a bench scale. The limitation of the π-complexation-based sorbents is lack of selectivity, because other olefinic and aromatic hydrocarbons in liquid transportation fuel can also be absorbed via π-complexation, thereby causing difficulty in maintaining a fairly good octane and cetane number. The use of clay minerals in sorbents has been studied as clay minerals occur abundantly in nature and because of their high surface area, adsorptive and ion-exchange properties, they have been studied for catalytic applications,5,6 in soil chemistry as well as effective sorbents for various water pollutants and are particularly cost effective. Clay-supported copper (II) and iron (III) nitrates (Claycop and Clayfen) have demonstrated unique oxidizing properties in organic synthesis. In Claycop and Clayfen, the copper and iron nitrates are immobilized on the clay surface by strict preparation techniques including dissolution of the metal nitrates in acetone, followed by careful removal of the solvent under reduced pressure to immobilize the salts on the clay surface. Clayfen is unstable, decomposes rapidly with the evolution of gas. The clay and metal nitrates are present in about a 1:1 weight ratio for both Claycop and Clayfen.
Accordingly, there is a need for a composition and method for effective and cost-efficient desulfurization of liquid hydrocarbon fuels including transportation fuels. There is another need for a composition and method to substantially reduce the refractory sulfur-containing aromatics from liquid hydrocarbon fuels. There is also a need for a composition and method for post-treating technology (polishing) to selectively remove residual sulfur compounds from liquid hydrocarbon fuels after a hydrodesulfurization (HDS) process to produce ultra low sulfur fuels. There is a further need for a composition and method that substantially removes a maximum amount of the sulfur components from fuels while substantially maintaining olefin and other hydrocarbon saturation to provide a good octane number. There is a still further need for a composition and method that is less expensive and more selective than the existing hydrodesulfurization (HDS) process and the transition metal exchanged zeolite sorbents. An additional need for a novel composition and method exists to produce low/ultra low sulfur liquid hydrocarbon fuels. A still further need exists for a sulfur removal composition and method that can be tailored for both refinery and non-refinery applications including on-site and on-board sulfur removal for fuel cell applications. The present invention fulfills these needs and provides other related advantages.